This invention relates to a bolt for transmitting axial load and a method for determining the magnitude of the axial load in the bolt. In particular, the invention concerns a bolt of the type having means for measuring the actual load transmitted by the bolt.
In order to obtain a tension measuring or indicating means that is simple and inexpensive and by which a direct and accurate measurement of the bolt tension is obtained, there have been suggested a number of bolt designs by which the elongation of the bolt or the elastic deformation of the bolt head are used to obtain such indication.
One such prior art bolt design is disclosed in U.S. Pat. No. 3,886,840, and comprises a weakened bolt head provided with a coaxial indicator pin and a washer fitted in a recess in the head. The bolt tension is indicated as the relative axial position of the pin and the washer caused by deformation of the bolt head. This prior art bolt design is disadvantageous due to the high manufacturing cost of the bolt, which partly refers to the introduction of an indicator pin and washer assembly and partly to the fact that a mass produced standard dimension bolt cannot possibly be used, since the head of such a bolt is too small to lodge such assembly.
In U.S. Pat. No. 3,718,066 there is disclosed another prior art bolt design by which the bolt tension may be indicated. For that purpose, the bolt head is provided with a photoelastic strip which is exposed to bending stresses as the bolt head is elastically deformed and generates a number of fringes responsive to the tension in the bolt. Thus generated fringes are visible in polarized light only and has to be counted manually. This means that the bolt tension is not measurable during tightening of the bolt if using a socket wrench since the photoelastic strip is then not visible. Moreover, the photoelastic strip has very little resistance against mechanical damage which may jeopardize later tension checks.
In U.S. Pat. No. 3,788,186, there is disclosed still another prior art tension bolt provided with stress indicating means. The bolt head is provided with one or more cavities in which are inserted piecies of brittle material. Due to the axial load in the bolt, the head is deformed such that its upper region is compressed, and, when reaching a certain tension level in the bolt, the brittle pieces are crushed which is easily noticable. This stress indicating means, however, is disadvantageous in that it does not enable a later bolt tension check. Neither does it indicate decreasing bolt tension.
An object of the present invention is to avoid the problems concerned with the above related prior art devices and to provide a simple and inexpensive bolt for transmitting axial load of a magnitude indicated by a direct measurable elastic deformation of the bolt head.